The invention relates to a process for the production of organically modified precipitated silicic acids and silicates which are suitable particularly for delustering films of lacquers.
It has been known to use synthetic silicon dioxide-containing as substances as dulling agents in lacquer systems. The silicic acids present in such lacquer systems--and that pertains equally to xerogels and aerogels--very often tend to settle at the bottom of the lacquer and, upon prolonged storage, form a hard deposit, which can be distributed only with the greatest difficulty or not at all in the lacquer through simple stirring. The tendency to form hard deposits at the bottom of the lacquers is particularly marked when the lacquers contain polar solvent components, such as the lower alcohols.
There have been many attempts to improve the behavior of silicic acids in lacquers and to minimize the tendency to form deposits. Included among these are the impregnation of silicic gels with inorganic fluorides and complex fluorides, with C.sub.1 -C.sub.5 alcohols and with organic silicates (ester of the ortho silicic acid), and, finally, with a microcrystalline, inert petroleum wax. In the latter case, a precipitated silicic acid gel or a hydrogel is coated with a fusible wax, whereby the coated silicic acid has a relatively high content of wax of 15 - 30% by weight, which is necessary for the achievement of a favorable deposit behavior. To maintain the strength and hardness of the films of lacquer, as little was as possible should be put into the silicic acid.
The above-mentioned treatments for an improvement of the deposit behavior of silicic acids in lacquers through their impregnation with fluorides and organic silicates, leads to improvements in the redispersibility of the deposits on the bottom of the lacquer containers. All of the silicic gels impregnated with wax in practically all known lacquer systems can be dispersed smoothly in the lacquer without difficulties through simple agitating or stirring.
However, the dulling effect on a lacquer of a silica gel impregnated with wax in such a manner is clearly decreased in comparison with other commercial dulling silicic acids. Another disadvantage of such wax-impregnated gels is the low resistance to scratching imparted to the films of dulled lacquer. This undesirable property is due to the relatively high content (15 - 30% by weight) of natural wax present in the lacquer films. Furthermore, such films of lacquer very soon become lustrous through mechanical wear.
While the wax impregnated silicic acids do show a certain fineness in particle size, they no longer satisfy the requirements of modern lacquer and pigment technology and its increasing demands with respect to lacquered surfaces, especially for surfaces coated with relatively thin films of pigmented lacquer.
It is possible with the use of modern techniques, such as micronizing, to produce silicic acid of a very fine particle size. However, experience has shown that with an increase in the fineness of the silicic acid, the quality of the lacquer surface improves but the capacity for dulling by the silicic acid clearly decreases. At the same time, the thickening effect which is undesirable and which can be the cause for breakdowns when the lacquer is sprayed, increases greatly.
The invention started with the task of finding a process for the production of organically modified, precipitated silicic acids and silicates through impregnation of the silicic acids and silicates with waxes or plastics to form products with improved dulling and deposit properties in clear varnishes (lacquers) or pigmented lacquers of all kinds. Such wax-impregnated compounds can be dispersed in the lacquers through simple stirring with a high speed mixer and remain suspended in the above-mentioned lacquers even during prolonged storage, or can be redispersed in the lacquer without difficulty in the event some of them do settle or deposit after storage.
The characterizing part of the invention is to be seen in the fact that during and/or after precipitation of silica gels or silicic acid or silicates from alkali silicate solutions in the presence of acids or metal salts, an aqueous emulsion of an inert, colorless, high-melting wax or plastic is added, and the mixture is subsequently filtered, the filter cake is washed, dried, and finely ground.
According to the invention, the impregnation of the silicic acid or silicates can take place either by addition of the wax emulsion during the precipitation process or, after the precipitation of the silicic acid or silicates, by adding the proper quantity of a wax emulsion to the precipitation emulsion. It is also possible to add a part of the wax emulsion during, and another part after precipitation of the silicic acid or silicates.
Furthermore, one can subject a salt-free dough of silicic acid, or a dried, finely dispersed silicic acid or silicate dispersed in water, to an after treatment, by treating them additionally with an aqueous wax emulsion and precipitating the emulsified compound onto the precipitated substance by suitable measures, such as change of temperature or of the pH value of the suspension.
The addition of the wax emulsion to the precipitation suspension of the silicic acid or silicate, or to the redispersed silicic acid or silicate is not limited to a special type of precipitated silica or silicate precipitated under certain precipitation conditions (pH value, temperature, SiO.sub.2 concentrations and precipitation speed). Basically, through the process according to the invention, a wide range of types of silicic gel or silicic acid, or silicate suspension can be impregnated with wax in suspension as a result of which the deposit behavior of the modified silicic acid or silicates in lacquers, as well as the resistance to scratching of the films of lacquers, is considerably improved as compared to other commercial, wax-modified dulling silicic acids.
It will be understood that the terms "precipitated silicic acids or silicates" include such substances that are formed directly by precipitation from alkali silicate solutions, i.e. preferably water glass and mineral acids or metal salt solutions. In this manner one can very easily obtain powdery, soft, and quickly dispersible dulling agents.